In recent years, semiconductor integrated circuits, micromachines, and the like have been processed in highly fine patterns. Therefore, a highly accurate process and a process to form a high aspect ratio pattern are required. In the fields of such processing, there has widely been used a plasma etching apparatus.
As a plasma etching apparatus, there has been known a reactive ion etching (RIE) apparatus which generates various kinds of particles including positive ions and radicals. The positive ions or the radicals are applied to a workpiece to etch the workpiece.
In an etching process utilizing such an RIE apparatus, there have been problems that high accuracy and high selectivity cannot be achieved simultaneously and etching profile irregularities are caused by charge build-up. The selectivity is a ratio of the etched depth in a workpiece to the etched depth in a mask or an underlying material. Specifically, when a workpiece is etched by x μm and a mask protecting the workpiece is etched by y μm, the selectivity s is expressed by s=x/y. In the case of a higher selectivity, the mask is less damaged and the workpiece can be etched to form a pattern having a high aspect ratio.
In order to enhance the selectivity, a combination of gases which can deposit on the mask or the underlying material but can etch the workpiece has been used in the conventional etching process. Further, radicals deposit onto the surface of the workpiece to form a sidewall passivation layer. If the sidewall passivation layer is excessively formed on the surface of the workpiece, then the surface of the workpiece is processed into a tapered shape, so that dimensional accuracy is lowered in the etching process. When a combination of Cl2 gas and O2 gas is used in the conventional etching process, the selectivity of Si/SiO2 is at most about 100. Thus, this combination of gases can achieve a higher selectivity than other combinations of gases. However, devices having a pattern smaller than 0.1 μm have been required to be processed with high accuracy and a selectivity higher than 300. Particularly, it will be the future task to simultaneously achieve a higher selectivity over an underlying layer of a gate oxide film and no residue at step portions for isolation.
The etching profile irregularities are caused by the difference between the behavior of electrons and that of positive ions. Specifically, the etching profile irregularities, i.e., notches, are produced at sidewalls defining stripes of a fine pattern. When the etching process is performed with a low energy ion beam, electrons are decelerated within the fine pattern by a negative self-bias potential on the substrate, and trapped near a resist. On the other hand, ions are accelerated and delivered to the underlying layer of the oxide film to develop positive charge build-up on the substrate. However, at the outside of the fine pattern, charge build-up doesn't occur because the same amounts of electrons and ions are delivered thereto. Thus, a potential difference is produced between the inside and outside of the fine pattern, so that the trajectories of the ions are curved to produce the notches.